Integrated dual swept source for OCT medical imaging

ABSTRACT

An optical coherence analysis system comprising: a first swept source that generates a first optical signal that is tuned over a first spectral scan band, a second swept source that generates a second optical signal that is tuned over a second spectral scan band, a combiner for combining the first optical signal and the second optical signal for form a combined optical signal, an interferometer for dividing the combined optical signal between a reference arm leading to a reference reflector and a sample arm leading to a sample, and a detector system for detecting an interference signal generated from the combined optical signal from the reference arm and from the sample arm. In embodiments, the swept sources are tunable lasers that have shared laser cavities.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.12/980,144, filed on Dec. 28, 2010, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

Optical coherence analysis relies on the use of the interferencephenomena between a reference wave and an experimental wave or betweentwo parts of an experimental wave to measure distances and thicknesses,and calculate indices of refraction of a sample. Optical CoherenceTomography (OCT) is one example technology that is used to performusually high-resolution cross sectional imaging. It is often applied toimaging biological tissue structures, for example, on microscopic scalesin real time. Optical waves are reflected from an object or sample and acomputer produces images of cross sections of the object by usinginformation on how the waves are changed upon reflection.

The original OCT imaging technique was time-domain OCT (TD-OCT), whichused a movable reference mirror in a Michelson interferometerarrangement. In order to increase performance, variants of thistechnique have been developed using two wavelengths in so-called dualband OCT systems.

In parallel Fourier domain OCT (FD-OCT) techniques have been developed.One example is time-encoded OCT, which uses a wavelength swept sourceand a single detector: it is sometimes referred to as time-encodedFD-OCT (TEFD-OCT) or swept source OCT. Another example is spectrumencoded OCT, which uses a broadband source and spectrally resolvingdetector system and is sometimes referred to as spectrum-encoded FD-OCTor SEFD-OCT.

These various OCT techniques offer different performancecharacteristics. FD-OCT has advantages over TD-OCT in speed andsignal-to-noise ratio (SNR). Of the two FD-OCT techniques, swept-sourceOCT has distinct advantages over spectrum-encoded FD-OCT because of itscapability of balanced and polarization diversity detection; it hasadvantages as well for imaging in wavelength regions where inexpensiveand fast detector arrays, which are typically required for SEFD-OCT, arenot available.

Swept source OCT has advantages in some additional respects. Thespectral components are not encoded by spatial separation, but they areencoded in time. The spectrum is either filtered or generated insuccessive frequency steps and reconstructed beforeFourier-transformation. Using the frequency scanning swept source, theoptical configuration becomes less complex but the critical performancecharacteristics now reside in the source and especially its tuning speedand accuracy.

The swept sources for TEFD-OCT systems have been typically tunablelasers. The advantages of tunable lasers include high spectralbrightness and relatively simple optical designs. The typical tunablelaser is constructed from a gain medium, such as a semiconductor opticalamplifier (SOA) and a tunable filter such as a rotating grating, gratingwith a rotating mirror, or a Fabry-Perot tunable filter. Currently, someof the highest speed TEFD-OCT lasers are based on the laser designsdescribed in U.S. Pat. No. 7,415,049 B1, entitled Laser with TiltedMulti Spatial Mode Resonator Tuning Element, by D. Flanders, M.Kuznetsov and W. Atia. This highly integrated design allows for a shortlaser cavity that keeps the round-trip optical travel times within thelaser short so that the laser is fundamentally capable of high speedtuning. Secondly, the use of micro-electro-mechanical system (MEMS)Fabry-Perot tunable filters combines the capability for wide spectralscan bands with the low mass, high mechanical resonant frequencydeflectable MEMS membranes that have the capacity for high speed tuning.

Another class of swept sources that has the potential to avoid inherentdrawbacks of tunable lasers is filtered amplified spontaneous emission(ASE) sources that combine a broadband light source, typically a sourcethat generates light by ASE, with tunable filters and amplifiers.

Some of the highest speed devices based on filtered ASE sources aredescribed in U.S. Pat. No. 7,061,618 B2, entitled IntegratedSpectroscopy System, by W. Atia, D. Flanders P. Kotidis, and M.Kuznetsov, which describes spectroscopy engines for diffuse reflectancespectroscopy and other spectroscopic applications. A number of variantsof the filtered ASE swept source are described, including amplifiedversions and versions with tracking filters.

More recently Eigenwillig, et al. have proposed a variant configurationof the filtered ASE source in an article entitled “Wavelength swept ASEsource”, Conference Title: Optical Coherence Tomography and CoherenceTechniques IV, Munich, Germany, Proc. SPIE 7372, 73720O (Jul. 13, 2009).The article describes an SOA functioning both as an ASE source and firstamplification stage. Two Fabry-Perot tunable filters are used in aprimary-tracking filter arrangement, which are followed by a second SOAamplification stage. Also, U.S. patent application Ser. No. 12/553,295,filed on Sep. 3, 2009, entitled Filtered ASE Swept Source for OCTMedical Imaging, by D. Flanders, W. Atia, and M. Kuznetsov, which isincorporated herein in its entirety by this reference, lays out variousintegrated, high speed filtered ASE swept source configurations. U.S.patent application Ser. No. 12/776,373, entitled ASE Swept Source withSelf-Tracking Filter for OCT Medical Imaging, filed on May 8, 2010, bythe same inventors, outlines still further configurations that rely onthe use of a self-tracking filter arrangement that can improveperformance both in terms of speed and linewidth, among other things,and which is also incorporated herein in its entirety by this reference.

SUMMARY OF THE INVENTION

Aspects of the present invention are directed to swept source systemsthat include multiple swept sources. It further concerns opticalcoherence tomography systems that incorporate and are compatible withsuch swept source systems.

In general, according to one aspect, the invention features a lasersystem that comprises multiple laser sources. Specifically a first lasersource generates a first tunable optical signal that is tuned over afirst spectral scan band, the first laser source having first lasercavity defined by a first reflector and a shared reflector, the firstlaser source including a first gain element for amplifying light in thefirst laser cavity and a first tuning element for dictating a wavelengthof the first optical signal. A second laser source generates a secondtunable optical signal that is tuned over a second spectral scan band,the second laser source having a second laser cavity defined by a secondreflector and the shared reflector, the second laser source including asecond gain element for amplifying light in the second laser cavity anda second tuning element for dictating a wavelength of the second opticalsignal. An intracavity combiner located between the shared reflector andeach of the first reflector and second reflector couples light to theshared reflector and back into the first laser cavity and the secondlaser cavity.

In embodiments, a combined optical signal including the first opticalsignal and the second optical signal is extracted through the sharedreflector. Also, at least one birefringence compensation element ispreferably provided in at least one of the first laser cavity and thesecond laser cavity for controlling a polarization of light returning tothe first gain element and/or the second gain element.

Preferably, the intracavity combiner comprises polarization beamsplitter for dividing light returning from the shared reflector betweenthe first gain element and the second gain element. Here, a polarizationrotation element in one of the first laser cavity and the second lasercavity is useful for rotating the polarization of light received by thepolarization beam splitter. However, a polarization rotation element isprovided in each of the first laser cavity and the second laser cavityfor rotating the polarization of light received by the polarization beamsplitter, in other examples. A combined optical signal including thefirst optical signal and the second optical signal can be extractedthrough the polarization beam splitter.

In other embodiments, the intracavity combiner comprises beam splitterfor dividing light returning from the shared reflector between the firstgain element and the second gain element. In other implementations, WDMcombiners and beam switches are used.

In the typical application, the laser system is used in an OCT systemthat comprises an interferometer for dividing a combined optical signal,including the first tunable optical signal and the second tunableoptical signal, between a reference arm leading to a reference reflectorand a sample arm leading to a sample and a detector system for detectingan interference signal generated from the combined optical signal fromthe reference arm and from the sample arm.

Depending on the embodiment, the first spectral scan band and the secondspectral scan band are substantially the same, non-overlapping spectralscan bands, or contiguous spectral scan bands.

In general, according to another aspect, the invention features anoptical coherence analysis system that comprises a first swept sourcethat generates a first optical signal that is tuned over a firstspectral scan band, a second swept source that generates a secondoptical signal that is tuned over a second spectral scan band, acombiner for combining the first optical signal and the second opticalsignal for form a combined optical signal, an interferometer fordividing the combined optical signal between a reference arm leading toa reference reflector and a sample arm leading to a sample, a multichannel k-clock system for receiving the combined optical signal andseparately detecting the first optical signal to generate a first clockand the second optical signal to generate a second clock, and a detectorsystem for detecting an interference signal generated from the combinedoptical signal from the reference arm and from the sample arm inresponse to the first clock and the second clock.

In general, according to still another aspect, the invention features anoptical coherence analysis system comprising: a first swept source thatgenerates a first optical signal that is tuned over a first spectralscan band, a second swept source that generates a second optical signalthat is tuned over a second spectral scan band, a combiner for combiningthe first optical signal and the second optical signal to form acombined optical signal, an interferometer for dividing the combinedoptical signal between a reference arm leading to a reference reflectorand a sample arm leading to a sample, a coherence analysis detectorsystem for detecting an interference signal generated from the firstoptical signal, and a spectral analysis detector for detecting thesecond optical signal after interaction with the sample.

In general, according to still another aspect, the invention features anoptical coherence analysis system comprising a micro optical bench, afirst ASE swept source that generates a first optical signal on themicro optical bench, a second ASE swept source that generates a secondoptical signal on the micro optical bench, a combiner, on the opticalbench, for combining the first optical signal and the second opticalsignal for form a combined optical signal, an interferometer fordividing the combined optical signal between a reference arm leading toa reference reflector and a sample arm leading to a sample, a k-clocksystem, on the micro optical bench, for receiving the combined opticalsignal to generating a clock, and a detector system for detecting aninterference signal generated from the combined optical signal from thereference arm and from the sample arm in response to the clock signal.

In general, according to still another aspect, the invention features anoptical coherence analysis system comprising a first swept source thatgenerates a first optical signal that is tuned over a first spectralscan band, a second swept source that generates a second optical signalthat is tuned over a second spectral scan band, a combiner for combiningthe first optical signal and the second optical signal to form acombined optical signal, an interferometer for dividing the combinedoptical signal between a reference arm leading to a reference reflectorand a sample arm leading to a sample, a coherence analysis detectorsystem for detecting an interference signal generated from the firstoptical signal and the second optical signal, the coherence analysisdetector system comprising WDM filters for separating the combinedoptical signals from the reference arm and the sample arm between afirst detector subsystem that generates interference signals from thefirst optical signal and a second detector subsystem that generatesinterferences signals from the second optical signal.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a schematic view of an OCT system with a dual swept sourceaccording to an embodiment of the invention;

FIG. 2A is a schematic view of an OCT system with a dual swept sourceaccording to another embodiment of the invention incorporating awavelength and/or polarization diversity optical receiver;

FIG. 2B is a schematic view of an OCT system with a dual swept sourceaccording to another embodiment of the invention incorporating apolarization sensitive optical receiver functioning at multiplewavelengths;

FIG. 3 is a schematic view of an OCT system with a dual swept sourceaccording to another embodiment of the invention incorporating spectralanalysis functionality;

FIG. 4 is a block diagram of a dual filtered ASE swept optical source;

FIG. 5 is a block diagram of a dual filtered ASE swept optical sourcewith an integrated k-clock system;

FIG. 6 is a schematic diagram of a dual laser swept optical source;

FIG. 7 is a schematic diagram of a dual laser swept optical source witha shared optical cavity according to the present invention;

FIG. 8 is a diagram of a birefringence controller used in the sweptsource optical system;

FIG. 9 is a schematic diagram of a dual laser swept optical source witha shared optical cavity according to another embodiment;

FIG. 10 is a schematic diagram of a dual laser swept optical source witha shared optical cavity according to still another embodiment;

FIG. 11 is a schematic diagram of a dual laser swept optical source withaccording to still another embodiment with birefringence controllers foreach individual swept source;

FIG. 12 is a top plan scale drawing of the dual laser swept opticalsource;

FIG. 13 is a top plan view of a multichannel k-clock system according tothe present invention;

FIGS. 14A-14F are plots of wavelength as a function of time showingswept source scanning, with FIG. 14A showing conventional scanning. FIG.14B showing time multiplexed scanning over a common scan band, FIG. 14Cshowing time multiplexed scanning over a common scan band withpolarization diversity, FIG. 14D showing time multiplexed scanning overa different contiguous scan bands, FIG. 14E showing time multiplexedscanning over different, non-overlapping scan bands with a guard band,and FIG. 14F showing simultaneous scanning over a different scan bands;

FIG. 15 is a block diagram showing an OCT system with a four sweptsources;

FIGS. 16A and 16B are plots of wavelength as a function of time showingswept source scanning, simultaneously over two and four different scanbands, respectively; and

FIG. 17 is plot of wavelength as a function of time showing swept sourcescanning, simultaneously over two different scan bands with differentscan rates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an optical coherence analysis system 300 using theintegrated dual swept source 100, which has been constructed accordingto the principles of the present invention.

The integrated dual swept source system 100 generates a combined opticalsignal on optical fiber 320 that is transmitted to interferometer 50. Inthe preferred embodiment, this combined optical signal is a tunableoptical signal that scans over a combined scanband with a narrowbandemission.

The integrated dual swept source system 100 comprises at least a firstswept source 100-1 and a second swept source 100-2. Each of theseindividual swept sources generates respective tunable optical signals, afirst optical signal and a second optical signal. The first opticalsignal and the second optical signal are combined into the combinedoptical signal by a combiner 200 and coupled onto the optical fiber 320.

In one embodiment, the first swept source 100-1 and a second sweptsource 100-2 of the integrated dual swept source system 100 are operatedin a time-multiplexed “ping-pong” fashion. At any given instant, thecombined optical signal on optical fiber 320 is derived from only one ofthe swept sources 100-1, 100-2.

Preferably, the integrated dual swept source system 100 also furthercomprises a k-clock module 250. The k-clock module generates a clockingsignal at equally spaced optical frequency increments as the combinedtunable optical signal is timed over the combined scan band.

In the current embodiment, a Mach-Zehnder-type interferometer 50 is usedto analyze the optical signals from the sample 340. The combined tunablesignal from the swept source module 100 is transmitted on fiber 320 to a90/10 optical coupler 322. The combined tunable signal is divided by thecoupler 322 between a reference arm 326 and a sample arm 324 of thesystem.

The optical fiber of the reference arm 326 terminates at the fiberendface 328. The light exiting from the reference arm fiber endface 328is collimated by a lens 330 and then reflected by a mirror 332 to returnback, in some exemplary implementations.

The external mirror 332 has an adjustable fiber to mirror distance (seearrow 334), in one example. This distance determines the depth rangebeing imaged, i.e. the position in the sample 340 of the zero pathlength difference between the reference arm 326 and the sample arm 324.The distance is adjusted for different sampling probes and/or imagedsamples. Light returning from the reference mirror 332 is returned to areference arm circulator 342 and directed to a 50/50 fiber coupler 346.

The fiber on the sample arm 324 terminates at the sample arm probe 336.The exiting light is focused by the probe 336 onto the sample 340. Lightreturning from the sample 340 is returned to a sample arm circulator 341and directed to the 50/50 fiber coupler 346. The reference arm signaland the sample arm signal are combined in the fiber coupler 346 togenerate an interference signal. The interference signal is detected bya balanced receiver, comprising two detectors 348, at each of theoutputs of the fiber coupler 346. The electronic interference signalfrom the balanced receiver 348 is amplified by amplifier 350.

In one mode of operation, the first swept source 100-1 and a secondswept source 100-2 are operated in a time-multiplexed “ping-pong”fashion. Only, a single channel receiver is required to detect theinterference signal.

An analog to digital converter system 315 is used to sample theinterference signal output from the amplifier 350. Frequency clock andsweep trigger signals derived from the k-clock module 250 of the dualswept source 100 are used by the analog to digital converter system 315to synchronize system data acquisition with the frequency tuning of theswept source system 100.

Once a complete data set has been collected from the sample 340 byspatially raster scanning the focused probe beam point over the sample,in a Cartesian geometry, x-y, fashion or a cylindrical geometry theta-zfashion, and the spectral response at each one of these points isgenerated from the frequency tuning of the dual swept source 100, thedigital signal processor 380 performs a Fourier transform on the data inorder to reconstruct the image and perform a 2D or 3D tomographicreconstruction of the sample 340. This information generated by thedigital signal processor 380 can then be displayed on a video monitor.

In one application, the probe 336 is inserted into blood vessels andused to scan the inner wall of arteries and veins. In other examples,other analysis modalities are included in the probe such asintravascular ultrasound (IVUS), forward looking IVUS (FLIVUS),high-intensity focused ultrasound (HIFU), pressure sensing wires andimage guided therapeutic devices.

FIG. 2A shows an optical coherence analysis system 300 that provides forpolarization sensitive coherence analysis.

In this second embodiment, the dual swept source 100 and specificallythe first swept source 100-1 and the second swept source 100-2 arepreferably operated in a time multiplexed fashion.

The detector system has the capacity to separate the interference signalinto two orthogonal polarizations. Polarization beam splitters 362 and364 separate the polarizations which are then detected by two balanceddetectors 348-1 and 348-2. Separate amplifiers 350-1 and 350-2 areprovided, and the analog to digital conversion board 315 includes twochannels to enable simultaneous detection of the output of amplifiers350-1 and 350-2.

FIG. 2B shows an optical coherence analysis system 300 that provides forpolarization sensitive detection at two wavelengths simultaneously.

Here, the dual swept source 100 and specifically the first swept source100-1 and the second swept source 100-2 are not necessarily operated ina time multiplexed fashion. At least, it is not a requirement of thearchitecture that they are operated in this fashion. Instead, the firsttunable optical signal from the first swept source 100-1 and the secondtunable optical signal from the second swept source 100-2 are separatedin wavelength. As a result, the combined optical signal on optical fiber320 in one implementation of this embodiment is at any instant acombination of the first tunable optical signal and the second tunableoptical signal, which are tunable over different spectral bands.

The detector system has the capacity to separate the reference andsample arms signals into portions derived from first tunable opticalsignal and the second tunable optical signal prior to detection. A firstwavelength division multiplexing (WDM) splitter 384 and a second WDMsplitter 386 are used to separate the spectral components of the signalsfrom the circulators 341, 342. The separate spectral components are sentto two separate detector subsystems 388, 390. In this implementation, itis not a requirement of the architecture that the first swept source100-1 and the second swept source 100-2 are operated in this fashion.Instead, the first tunable optical signal from the first swept source100-1 and the second tunable optical signal from the second swept source100-2 are separated in wavelength. The two separate detector subsystems388, 390 are used to separately detect the interference signals thatresult from the first tunable optical signal from the first swept source100-1 and the second tunable optical signal that is generated by thesecond swept source 100-2.

In more detail, each of the first detector subsystem 388 and the seconddetector subsystem 390 has a fiber coupler 346 for generatinginterference signals. Polarization beam splitters 362 and 364 thenseparate the interference signals into the separate orthogonalpolarizations to enable polarization sensitive detection. Two balanceddetectors 348-1 and 348-2 in each the detector subsystems 388, 390separately detect the interference signals that result from the firsttunable optical signal from the first swept source 100-1 and the secondtunable optical signal that is generated by the second swept source100-2. Separate amplifiers 350-1 and 350-2 are provided, and the analogto digital conversion board 315 includes four channels to enablesimultaneous detection of the output of amplifiers 350-1 and 350-2 fromeach the detector subsystems 388, 390.

It should be noted that while the illustrated embodiment is shown withtwo swept-sources and two detection channels, more than two sweptsources and more than two corresponding detection channels are used instill further embodiments. In such implementations, a WDM combiner 200is used to combine multiple swept sources 100-1 to 100-n, such as n=4 or6 or more sources. WDM splitters 384, 386 are then used to separate outthe signals associated with the different swept sources for detection atmultiple detector subsystems 388, 390.

FIG. 3 shows an optical coherence analysis system 300 that has beenconstructed according to a third embodiment of the present invention.

This third embodiment includes the capability to perform spectroscopicanalysis on the sample 340.

In more detail, in the preferred embodiment, two optical fibers areprovided to the probe 336. Optical fiber 350 transmits the combinedsignal including first tunable optical signal and the second tunableoptical signal to the probe 336, which directs the signals to the sample340. Light returning from the sample 340 that is used for opticalcoherence analysis returns on optical fiber 350 to circulator 341. Thisreturning light is processed as described in the previous embodiments togenerate an optical coherence analysis of the sample 340.

In contrast, light that is used for spectral analysis of the sample 340is coupled from the probe on optical fiber 352. This spectral analysislight is detected by spectral analysis detector 356. In oneimplementation, a filter and collimator element 354 is used to directthe light onto the spectral analysis detector 356 and also possiblyremove any spectral components of the signal that are related to theoptical coherence analysis of the sample 340.

In one implementation, the first swept source 100-1 is used for opticalcoherence analysis. The second swept source 100-2 is used for spectralanalysis of the sample 340. Typically, these two swept sources willoperate with different spectral scan bands. In this implementation, thefilter and collimator element 354 is a WDM filter that transmits onlythe scanband generated by the second swept source 100-2.

In still a further implementation, the spectral analysis of the sample340 is performed at the same spectral regions as the optical coherenceanalysis. In this case the filter and collimator element 354 passes thespectral components associated with both the first swept source 100-1and the second swept source 100-2 and the detector 356 detects thespectral response of the sample 340 in a time multiplexed fashion.Alternatively, when the first swept source 100-1 in the second sweptsource 100-2 operate in different spectral scan bands, then the filterand collimator element 354 allows the light from only one of these scanbands to reach the detector 356 when they cannot be separated in time.

FIG. 4 shows a first embodiment of the dual swept optical source 100that has been constructed according to the principles of the presentinvention.

The first and the second swept sources 100-1 and 100-2 are each filteredamplified spontaneous emission sources. In the current embodiments theseASE swept sources 100-1 and 100-2 are any one of ASE swept sourcesdescribed in U.S. patent application Ser. No. 12/553,295, entitledFiltered ASE Swept Source for OCT Medical Imaging, filed on Sep. 3, 2009by Flanders, et al. or in U.S. patent application Ser. No. 12/776,373,entitled ASE Swept Source with Self-Tracking Filter for OCT MedicalImaging, filed on May 8, 2010. Additionally, other amplified ASE sweptsources could be used in still further examples. Nevertheless, thefollowing example is provided based on one of the swept sourceconfigurations of this incorporated document.

In more detail, the swept sources 100-1, 100-2 each comprise a broadbandsource 112-1, 112-2 that generates a broadband optical signal. Ingeneral, the broadband signal is characterized by a continuous spectrumthat extends in wavelength over at least 40 nanometers (nm) ofbandwidth, full width half maximum (FWHM). Typically, the continuousspectrum extends over at least 70 nm and preferably over 100 nm orgreater.

In the preferred embodiment, the broadband sources 112-1, 112-2 areelectrically pumped semiconductor chip gain media that are bonded orattached to a common bench B. Examples of the sources 112-1, 112-2include superluminescent light emitting diodes (SLED) and semiconductoroptical amplifiers (SOA). The material systems of the chips are selectedbased on the desired spectral operating range for each of the first andthe second swept sources 100-1 and 100-2. Common material systems arebased on III-V semiconductor materials, including binary materials, suchas GaN, GaAs, InP, GaSb, InAs, as well as ternary, quaternary, andpentenary alloys, such as InGaN, InAlGaN, InGaP, AlGaAs, InGaAs,GaInNAs, GaInNAsSb, AlInGaAs, InGaAsP, AlGaAsSb, AlGaInAsSb, AlAsSb,InGaSb, InAsSb, and InGaAsSb. Collectively, these material systemssupport operating wavelengths from about 400 nm to 2000 nm, includinglonger wavelength ranges extending into multiple micrometer wavelengths.Semiconductor quantum well and quantum dot gain regions are typicallyused to obtain especially wide gain and spectral emission bandwidths.Currently, edge-emitting chips are used although vertical cavity surfaceemitting laser (VCSEL) chips are used in different implementations.

The use of semiconductor chip gain media for the first and the secondswept sources 100-1 and 100-2 has advantages in terms of systemintegration since these semiconductor chips can be bonded to submountsthat in turn are directly bonded to the common bench B. Other possiblegain media can be used in other implementations, however. In theseexamples, the broadband signal is typically transmitted via opticalfiber to the bench B. Such examples include solid state gain media, suchas rare-earth (e.g., Yb, Er, Tm) doped bulk glass, waveguides or opticalfiber.

In these examples, the output facets of the chips or gainwaveguides/fibers are antireflection coated, and possibly angled, sothat the gain media do not lase but instead generate broadband light viaamplified spontaneous emission (ASE).

In some embodiments, the broadband sources 112-1, 112-2 have differentoperating or spectral emissions to form separate non-overlappingspectral bands. In other examples, the spectral bands of the broadbandsources 112-1, 112-2, are contiguous or potentially overlapping. As aresult, the broadband sources 112-1, 112-2 are in some examples chipsmade out of different material systems.

The use of sources with complementary spectral bands enables the dualswept source system 100 to generate a combined tunable optical signalthat covers a wider wavelength scanning range. This translates into ahigher spatial (depth) resolution for the imaging system. Alternatively,the two different spectral bands can provide complementary informationabout the sample, such as different penetration depth into the sampleand different sample image contrast. In still other embodiments,spectral bands are selected for the best optical coherence analysis anddifferent spectral bands are selected for the spectral analysis of thesample.

The use of two swept sources with substantially the same spectral bandbut multiplexed in alternating time slots in a “ping pong” fashionallows higher duty cycle for data acquisition and better frequencytuning linearity of each of the two sources. This “ping pong” dual laserapproach can be used advantageously instead of the standard opticalbuffering approach in high speed OCT, where two copies of the opticalsignal from a single swept source are time multiplexed after timedelaying one of the signal copies in a long length of optical fiber. Theadvantage of the dual source approach is that, unlike the case withoptical buffering, it does not require a long length of fiber with itspossible strong dispersion that strongly degrades imaging resolution.

Another major benefit of the ping-pong approach is the 2× higherattainable duty cycle, which relaxes the maximum data acquisitionfrequency requirements for a given scan range and scan speed by a factorof 2, allowing for lower cost data acquisition components or faster scanspeeds.

In other examples, the broadband sources 112-1, 112-2 generate broadbandsignals that have different polarizations, such as orthogonalpolarizations. The advantage here is that the optical signals from theseparate broadband sources provide polarization diversity sampleillumination and signal detection such as to eliminate polarizationsensitivity of the imaging system and eliminate polarization artifactsin the image. Alternatively, the two orthogonal polarizations can beused for polarization sensitive imaging that can enhance imagingcontrast of specific regions of interest in the sample.

The bench B is termed a micro-optical bench and is preferably less than20 millimeters (mm) in width and about 50 mm in length or less. Thissize enables the bench to be installed in a standard, or nearstandard-sized, butterfly or DIP (dual inline pin) hermetic package. Inone implementation, the bench B is fabricated from aluminum nitride. Athermoelectric cooler is disposed between the bench B and the package(attached/solder bonded both to the backside of the bench and innerbottom panel of the package) to control the temperature of the bench B.

The broadband optical signals from the broadband sources 112-1, 112-2are coupled to respective isolators 114-1, 114-2, which are preferablyalso bonded or attached to the bench B. These isolators 114-1, 114-2prevent feedback into the broadband sources 112-1, 112-2 that mightcause them to lase or otherwise change, e.g. produce ripple in, theemission spectrum of the broadband optical signal from the broadbandsources.

First-stage tunable filters 116-1, 116-2 function as tunable bandpassfilters to convert the broadband signals to narrow band tunable signals,the first and second tunable signals. In a current embodiment, thepassband of the first stage tunable filters have a full width halfmaximum (FWHM) bandwidths of less than 20 or 10 GigaHertz (GHz), and arepreferably 5 GHz or less. For spectroscopy this relatively narrowpassband yields high spectral resolution. For optical coherencetomography, this high spectral resolution implies long coherence lengthof the source and therefore enables imaging deeper into samples, forexample deeper than 5 mm. In lower performance applications, for exampleOCT imaging less than 1 mm deep into samples, broader FWHM passbands aresometimes appropriate, such as passbands of about 200 GHz or less.

The first-stage tunable filters 116-1, 116-2 are scanned over a firstspectral scan band and a second spectral scan band, respectively. Insome implementations, first spectral scan band and the second spectralscan bands are non-overlapping, contiguous, overlapping, or the samescan bands. Generally, the scan bands of first-stage tunable filters116-1, 116-2 are matched to the corresponding emission spectral bands ofthe respective broadband sources 112-1, 112-2.

In the current embodiment, the first stage tunable filters 116-1, 116-2are Fabry-Perot tunable filters that are fabricated usingmicro-electro-mechanical systems (MEMS) technology and are attached,such as directly solder bonded, to the bench B. Currently, the filters116-1, 116-2 are manufactured as described in U.S. Pat. No. 6,608,711 or6,373,632, which are incorporated herein by this reference. Acurved-flat resonator structure is used in which a generally flat mirrorand an opposed curved mirror define a filter optical cavity, the opticallength of which is modulated by electrostatic deflection of at least oneof the mirrors.

The tunable optical signals that are produced by the passband of thefirst stage tunable filters 116-1, 116-2 are amplified in first stageoptical amplifiers 120-1, 120-2 of a first amplification stage.Preferably the first stage optical amplifiers are SOA's withantireflection coated and angled front and rear facets, enablingintegration onto the bench B by attachment, typically via a submount.

Second isolators 118-1, 118-2 between the first stage tunable filters116-1, 116-2 and the first amplifiers 120-1, 120-2 prevent backreflections between the front facets of the first amplifiers 120-1,120-2 and the first stage tunable filters 116-1, 116-2 from causinglasing or other spectral ripple due to parasitic reflections betweenthese two elements. The second isolators 118-1, 118-2 are preferablyalso bonded or otherwise attached to the bench B.

The amplified tunable signals from the first stage amplifiers 120-1,120-2 are again passband filtered by second stage tunable filters 122-1,122-2. These second stage filters 122-1, 122-2 are preferably tunableMEMS Fabry-Perot filters as described previously and are preferably alsosimilarly solder-bonded or otherwise attached to the bench B. In someimplementations, the only difference between the first stage tunablefilters 116-1, 116-2 and the second stage tunable filters 122-1, 122-2are that the second stage tunable filters 122-1, 122-2 have slightlybroader passbands than the first stage tunable filters 116-1, 116-2,such as between 2 and 20 times broader in frequency. These second stagefilters 122-1, 122-2 are termed tracking filters because they arecontrolled to scan synchronously with the first stage tunable filters116-1, 116-2 and thus track tuning of the first stage filters. Thetracking filters function primarily to remove ASE noise introduced bythe first stage amplifiers 120-1, 120-2 and further spectrally shape andnarrow the tunable signal.

The synchronous tracking of the second stage tunable filters 122-1,122-2 with the first stage tunable filters 116-1, 116-2 is controlled bya tuning controller 125 that drives both filters of both stages andswept sources. Preferably, the tuning controller 125 spectrally centersthe passbands of the tracking tunable filters 122-1, 122-2 on passbandsof the first stage tunable filters 116-1, 116-2 and then tunes the twopassbands together over the scanband extending over the gain bands ofthe respective broadband sources 112-1, 112-2 and amplifiers 120-1,120-2, 126-1, 126-2.

Third isolators 121-1, 121-2 between the first stage amplifiers 120-1,120-2 and the second stage tunable filters 122-1, 122-2 prevent backreflections between the back facets of the first stage amplifiers 120-1,120-2 and the second stage tunable filters 122-1, 122-2 from causinglasing or other spectral ripple due to parasitic reflections betweenthese two elements and any other intervening elements such as lenses,not shown in this view. The third isolators 121-1, 121-2 are preferablyalso bonded or otherwise attached to the bench B.

The amplified tunable optical signals that are produced from the firststage optical amplifiers 120-1, 120-2 and filtered by the trackingfilters 122-1, 122-2 are again amplified in second amplifiers 126-1,126-2 of a second amplification stage. Preferably the second stageoptical amplifiers 126-1, 126-2 are also SOA's with antireflectioncoated and angled front and rear facets, enabling integration onto thebench B by attachment to it. In terms of control, the second stageoptical amplifiers 126-1, 126-2 are usually operated in saturation witha lower input saturation power to minimize broadband ASE contributionfrom this last gain stage.

Fourth isolators 124-1, 124-2 between the front facets of the secondstage amplifiers 126-1, 126-2 and the second stage tunable filters122-1, 122-2 prevent back reflections between the front facet of thesecond amplifiers 126-1, 126-2 and the second tunable filters 122-1,122-2 from causing lasing or other spectral ripple due to parasiticreflections between these elements. The fourth isolators 124-1, 124-2are preferably also bonded or otherwise attached to the bench B.

If required, still further gain stages can be used. In one example thirdSOAs, third amplification stage, are added. For other applicationshaving still higher power requirements, a rare-earth doped fiber gainstage is added after the second SOAs 126-1, 126-2.

The outputs of each second stage amplifiers 126-1, 126-2 are a firsttunable optical signal 128-1 and a second tunable optical signal 128-2.These optical signals are combined in a combiner stage 200 to form acombined optical signal 256 on optical fiber 320. In the preferredembodiment, the elements of the combiner stage 200 are implemented onand secured to the optical bench B.

In one example, the combiner 200 forms the combined optical signal 256using a WDM filter. In this implementation, a first spectral scan bandand the second spectral scan band are non-overlapping spectral scanbands. A fold mirror 254 directs the second tunable optical signal 128-2to a beam combining element 252, which is a WDM filter that reflectslight in the second spectral scan band and transmits light in the firstspectral scan band.

In another example, the first tunable optical signal 128-1 and thesecond tunable optical signal 128-2 have orthogonal polarizations. Thesignals are combined in the combined signal using a polarization beamcombiner as the beam combining element 252. Since this example relies onpolarization diversity, it works when the first spectral scan band andthe second spectral scan band are the same, overlapping, contiguous, andnon-overlapping. In order to produce the orthogonal polarizations aquarterwave plate 262 is typically added to the optical path of one ofthe sources.

In still another example, the first tunable optical signal 128-1 and thesecond tunable optical signal 128-2 are time multiplexed. Here, thesignals are combined in the combined signal using a beam switch as thebeam combining element 252 by alternately passing either the firsttunable optical signal 128-1 or the second tunable optical signal 128-2as the combined signal 256.

In still another example, the beam combining element 252 is a 50/50beamsplitter/combiner, where the second optical output of such combineris used in the input 260 to the K-clock module 250, which generates thek-clock signal for triggering analog-to-digital data acquisitionelectronics module 315.

FIG. 5 shows another embodiment of the dual swept optical source 100that has an integrated k-clock.

Particularly, the k-clock system 250 is integrated on the bench B alongwith the dual swept optical source 100. Preferably, the k-clock system250 and the components of the dual swept optical source 100 are furtherintegrated together within a common hermetic package 500.

The description presented above with respect to FIG. 4 generally appliesFIG. 5. However, this embodiment differs in that it includes theintegrated k-clock system 250.

In more detail, input beam 260 is the combined beam from the beamcombiner 252. It is reflected by a fold mirror 291. The light thenpasses through a beam splitter 290, which is preferably a 50/50 splitterto a clock etalon 292. Any light reflected by the splitter 290 isdirected to a beam dump component 294 that absorbs the light andprevents parasitic reflections in the hermetic package 500.

The clock etalon 292 functions as a spectral filter. Its spectralfeatures are periodic in frequency and spaced spectrally by a frequencyincrement related to the length and refractive index of the constituentmaterial of the clock etalon 292, which is fused silica in one example.The physical length of etalon 292 is L. The etalon can alternatively bemade of other high-index and transmissive materials such as silicon forcompactness, but the optical dispersion of the material may need to becompensated for with additional processing. Also, air-gap etalons, whichare nearly dispersionless, are another alternative. Still a furtheralternative is a free-space interferometer (e.g. a Michelson), whichalso is dispersionless and is adjusted by moving the relative positionsof the mirrors.

The contrast of the spectral features of the etalon 292 is determined bythe reflectivity of its opposed endfaces. In one example, reflectivityat the etalon endfaces is provided by the index of refractiondiscontinuity between the constituent material of the etalon and thesurrounding air, gas or vacuum. In other examples, the opposed endfacesare coated with metal or preferably dielectric stack mirrors to providehigher reflectivity and thus contrast to the periodic spectral features.

In the illustrated example, the clock etalon 292 is operated inreflection. The light returning from the clock etalon 292 and reflectedby beamsplitter 290 is detected by detector 298. The light detected bydetector 298 is characterized by drops and rises in power as thefrequency of the combined tunable optical signal scans through thereflective troughs/reflective peaks provided by the clock etalon 298.Light transmitted by the etalon 292 is collected by beam dump 295.

The implementation of the k-clock system 250 on the bench B providesadvantages in that a thermo electric cooler 299, that is also installedwithin the package 500, is used both to control the temperature of theswept source system 100 and temperature stabilize the k-clock system250. Additional advantages are the small overall size of the systemalong with robustness against shock since all of the components areinstalled on a common rigid bench B.

FIG. 6 shows another embodiment of the dual swept optical source 100,using laser sources, according to the principles of the presentinvention.

The first and the second laser swept sources 100-1 and 100-2 are eachpreferably lasers as described in incorporated U.S. Pat. No. 7,415,049B1.

In more detail, each of the tunable lasers sources 100-1, 100-2comprises a semiconductor gain chip 410-1, 410-2 that is paired with amicro-electro-mechanical (MEMS) angled reflective Fabry-Perot tunablefilter 412-1, 412-2 to create external cavity tunable laser (ECL) on acommon micro-optical bench B.

The semiconductor optical amplifier (SOA) chips 410-1, 410-2 are locatedwithin a laser cavity. In the current embodiment, input facets of theSOA chips 410-1, 410-2 are angled and anti-reflection (AR) coated,providing parallel beams from the two facets. The output facets arecoated to define one end of the laser cavities, in one example.

Each facet of the SOAs 410-1, 410-2 has associated lenses 414-1, 414-2,416-1, 416-2 that are used to couple the light exiting from either facetof the SOAs 410-1, 410-2. The first lenses 414-1, 414-2 couple the lightbetween the front facets of the SOAs 410-1, 410-2 and the respectivereflective Fabry-Perot tunable filter 412-1, 412-2. Light exiting outthe output or front facets of the SOAs 410-1, 410-2 is coupled by secondlenses 416-1, 416-2 to a combiner stage 200.

The angled reflective Fabry-Perot filters 412-1, 412-2 are amulti-spatial-mode tunable filters that provide angular dependentreflective spectral response back into the respective laser cavities.This phenomenon is discussed in more detail in incorporated U.S. Pat.No. 7,415,049 B1.

In one implementation, extender elements 415-1, 415-2 are added to thelaser cavities. These are transparent high refractive index material,such as fused silica or silicon or other transmissive material having arefractive index of about 1.5 or higher. Currently silicon is preferred.Both endfaces of the extender elements 415-1, 415-2 are antireflectioncoated. Further, the elements are preferably angled by between 1 and 10degrees relative to the optical axis of the cavities to further spoilany reflections from the endfaces from entering into the laser beamoptical axis. These extender elements 415-1, 415-2 are used to changethe optical distance between the laser intracavity spurious reflectorsand thus change the depth position of the spurious peak in the imagewhile not necessarily necessitating a change in the physical distancebetween the elements.

The combiner stage 200 forms a combined optical signal 256 from thefirst tunable optical signal 128-1 and the second tunable optical signal128-2 generated by the tunable lasers 100-1, 100-2. In the preferredembodiment, the elements of the combiner stage 200 are implemented onand secured to the optical bench B. The combined optical signal iscoupled by lens 264 into optical fiber 320 for transmission to theinterferometer of the OCT system.

In one example, the combiner 200 forms the combined optical signal 256using a WDM filter. In this implementation, a first spectral scan bandof first tunable laser 100-1 and the second spectral scan band of secondtunable laser 100-2 are non-overlapping spectral scan bands. A foldmirror 254 directs the second tunable optical signal 128-2 to a beamcombining element 252, which is a WDM filter that reflects light in thesecond spectral scan band and transmits light in the first spectral scanband.

In another example, the first tunable optical signal 128-1 and thesecond tunable optical signal 128-2 have orthogonal polarizations. Aquarter wave plate 262 is used to rotate the polarization of the secondtunable optical signal 128-2 from the second tunable laser 100-2. Inother examples, the SOA chips produce optical signals of orthogonalpolarizations. The signals are combined in the combined optical signal256 using a polarization beam combiner as the beam combining element252. Since this example relies on polarization diversity, it works whenthe first spectral scan band and the second spectral scan band the same,overlapping, contiguous, and non-overlapping.

In still another example, the first tunable optical signal 128-1 and thesecond tunable optical signal 128-2 are time multiplexed. Here, thesignals are combined in the combined signal 256 using a beam switch asthe beam combining element 252 by alternately passing either the firsttunable optical signal 128-1 or the second tunable optical signal 128-2as the combined signal 256.

In still another example, the beam combining element 252 is a 50/50beamsplitter/combiner, where the second optical output 260 of suchcombiner 252 is used in the K-clock module 250 for generating thek-clock signal for triggering analog-to-digital data acquisitionelectronics module 315.

FIG. 7 shows another embodiment of the dual swept optical source 100,using laser sources, according to the principles of the presentinvention.

In the embodiment of FIG. 6, the laser cavities are relatively short,extending between the tunable filters 412-1, 412-2 and the output facetsof the SOAs 410-1, 410-2. The embodiment of FIG. 7 provides for longertunable filter cavities by implementing a shared cavity arrangement inwhich tunable laser swept sources 100-1, 100-2 have a portion of thelaser cavity that is shared between them.

In more detail, in this embodiment, both facets of the SOAs 410-1, 410-2are antireflection coated. The optical cavities of each of the tunablelaser swept sources 100-1, 100-2 extend from their respective tunablefilters 412-1, 412-2 through the combiner stage 200 to a sharedreflector 270. This shared reflector 270 is a common reflector thatdefines an end of the laser cavity for both of the tunable laser sweptsources 100-1, 100-2.

In one implementation, the shared reflector 270 is a partiallyreflecting mirror formed on the end of a fiber pigtail 268 that isoptically coupled to the bench B. Light from both laser cavities oftunable laser swept sources 100-1, 100-2 is coupled into fiber pigtail268 via the combiner 200 and lens 264.

As described previously, there are a number of potential implementationsof the combiner stage 200. In one example, the combiner 200 combines thelight from the tunable lasers using a WDM filter. In thisimplementation, a first spectral scan band of first tunable laser 100-1and the second spectral scan band of second tunable laser 100-2 arenon-overlapping spectral scan bands. The fold mirror 254 directs thelight to a beam combining element 252, which is a WDM filter thatreflects light in the second spectral scan band and transmits light inthe first spectral scan band.

In another example, the first tunable optical signal 128-1 and thesecond tunable optical signal 128-2 have orthogonal polarizations. Aquarter wave plate 262 is used to rotate the polarization of the secondtunable optical signal 128-2 from the second tunable laser 100-2 ifrequired. In other examples, the SOA chips produce optical signals oforthogonal polarizations. The signals are combined in the combinedoptical signal 256 using a polarization beam combiner as the beamcombining element 252. Since this example relies on polarizationdiversity, it works when the first spectral scan band and the secondspectral scan band the same, overlapping, contiguous, andnon-overlapping.

In still another example, the first tunable optical signal 128-1 and thesecond tunable optical signal 128-2 are time multiplexed. Here, thelight from the two cavities is combined using a beam switch, in oneimplementation, as the beam combining element 252 by alternately passingeither the first tunable optical signal 128-1 or the second tunableoptical signal 128-2 as the combined signal 256.

In still another example, the beam combining element 252 is a 50/50beamsplitter/combiner. In this example, however, the beam splitter 252is an intra-cavity element. Thus, the loss associated with the elementis higher than in the previous embodiments.

One advantage of using the shared reflector 270 along with the fiberpigtail 268 is that it enables greater latitude in adjusting the lengthsof the optical cavities of the tunable laser swept sources 100-1, 100-2.Generally, shorter laser cavities translate to higher potential tuningspeeds. The round-trip travel time for the light in the laser cavitiesis kept low so that lasers can tune at very high speed. Short lasercavities, however, create problems in terms of the spacing of thelongitudinal cavity modes. That is, lasers can only produce light atinteger multiples of the cavity length since the light must oscillatewithin the cavities. Shorter cavities result in fewer and more widelyspaced modes. Fewer cavity modes result in greater mode hopping noise asthe laser is tuned over these cavity modes.

In the current embodiment, the optical length of each of the lasercavities for the tunable laser swept sources 100-1, 100-2 is adjustableby controlling the length of the fiber pigtail 268. In the currentembodiment, the fiber pigtail is between 3 and 9 centimeters, preferably60 millimeters (mm), long for lasers generating light with a centerwavelengths of 1310 and 1060 nanometers in length. This results in agood trade-off between the tuning speed of the tunable laser sweptsources 100-1, 100-2 while yielding acceptable mode hopping noise as thelasers are tuned over the their spectral scan bands.

In the preferred embodiment, an intracavity birefringence controlelement 565 is further provided. This element further functions as asplicing device that couples the pigtail 270 to optical fiber 320.

Typically, the SOA has gain in only one preferential polarizationdirection. As described previously, light reflected from the fibermirror coating 270 in the control element 565 serves as one mirror ofthe laser cavities. However, single-mode fiber (nonpolarization-maintaining), which is often preferred for OCTapplications. Polarization maintaining (PM) fiber can cause ghost imagesdue to some light coupling into the undesired fiber axis andexperiencing a longer time delay. Single mode fiber, however canarbitrarily rotate the polarization state of the light (calledbirefringence) and thus degrade laser feedback over the tuning range.Further, stress from soldering the fiber to the bench B or to any fiberfeed through 502 in package 500 further induces birefringence. In theworst-case instance, it can prevent lasing if the reflected light'sdirection of polarization ends up orthogonal to the preferredpolarization state of the SOAs 410-1, 410-2.

Inserting a birefringence control element 565 in the laser cavities oftunable laser swept sources 100-1, 100-2 compensates by properlyaligning the polarization state of the reflected light.

FIG. 8 shows one implementation of the birefringence control element 565that can provide a relatively short fiber cavity (from 3 to 8 cm) andallows for polarization alignment and fiber stability in a compactfixture, which is a mechanical fiber splicing device.

The fiber stub 268 is held by two points in the mechanical fibersplicing device. 1) solder at point 418 that connects fiber 268 to tube420, which is the coupler's ferrule; and 2) by the mechanical splicebetween fiber 268 and fiber 320 at the optical coating/combinedreflector 270.

One commercially available example of mechanical fiber splicing deviceis the 3M™ Fibrlok™ II Universal Optical Fiber Splice, which ismechanical splice device that allows the coupling the reflective (HR)coated fiber 270 to the output fiber 320. In other examples, a fusionsplice with an HR coating is used to couple fiber 268 to fiber 320.

This mechanical splice is fixed within the body 422 typically by anepoxy bond. A cylindrical holder 424 is fixed to the body 422 and has aninner bore into which the tube 420 is inserted. A set screw 426 enablesthe tube 420 to be rotated relative to the cylindrical holder 424 andthen be fixed to the holder 424 when the set screw 426 is tighteneddown. This allows a mechanical stress to be imparted to fiber 268 in itslength between solder 418 and the region of the shared reflector 270that is secured to body 422. This stress affects the birefringence ofthe fiber 268 and thus affects the polarization state of the lightwithin the laser cavities of the tunable laser swept sources 100-1,100-2.

In operation and typically in a manufacturing/calibration stage, withthe set screw 426 loosened, an operator twists the fiber 268 by rotatingthe body 422 about the stainless steel tube 420. The output of the dualswept source 100 is monitored by a detector connected to a high speedoscilloscope. Twisting of the fiber stub 268 induces polarizationchanges. When optimal operation is observed, characterized by a maximumpower output from the lasers and smooth tuning, i.e., reduced changes inpower as a function of frequency during tuning and the change infrequency as a function of time is linear or near linear during tuning,over the desired tuning range without any polarization fading, the setscrew 426 is tightened to fix the stress applied to the fiber 268. Thestainless steel tube 420 prevents any subsequent fiber movement and thusmaintains polarization stability. In this way, the mechanical fibersplicing device functions as a birefringence control element 565 that isused to compensate for other birefringence. This enables polarizationmatching to occur over the whole wavelength tuning range of the dualswept source 100 with any residual birefringence of the SMF fiber beingsmall.

FIG. 9 shows another embodiment of the dual swept optical source 100,using laser sources, according to the principles of the presentinvention.

It is similar to the embodiment shown in FIG. 7. It incorporates thebirefringence control element 565 and the fiber pigtail 268 that forms aportion of the shared optical cavity for the tunable laser swept sources100-1, 100-2. In this embodiment, the shared reflector 270 is preferablya 100% reflecting mirror. Light is instead extracted from the tunablelaser swept sources 100-1, 100-2 via the combiner 200.

In more detail, the combining element 252 in this embodiment is a beamsplitter. As a result, a portion of the light from both of the tunablelaser swept sources 100-1, 100-2 is transmitted to the shared reflector270. The light that is not reflected by the beam splitter 252 from thesecond tunable laser swept source 100-2 and the light that is reflectedby the beam splitter 252 from the first tunable laser swept source 100-1is collimated by output lens 272 and coupled into output optical fiber320 for transmission to the OCT interferometer.

FIG. 10 shows another embodiment of the dual swept optical source 100,using laser sources, according to the principles of the presentinvention.

This embodiment is similar to the embodiment discussed with respect toFIG. 9. It differs in that the combining element 252 is a polarizationbeam splitter. Further, two adjustable quarter wave plates are added inthe laser cavities of the tunable laser swept sources 100-1, 100-2.These adjustable quarter wave plates 274, 276 are located between thecombiner 200 and the SOAs 410-1, 410-1.

During manufacture, the adjustable quarter wave plates 274, 276, arerotated to adjust the polarization of the light transmitted to thecombiner 252. Changing their polarizations changes the degree to whichthey are reflected or transmitted by the polarization beam splitterelement 252. This has the effect of providing control over the lightthat is coupled from the optical cavities of the tunable laser sweptsources 100-1, 100-2 to output lens 272 and into the optical fiber 320that transmits the combined tunable optical signal to the OCTinterferometer.

In the preferred embodiment, the adjustable quarter wave plates 274, 276are adjusted such that approximately 50-99 percent of the light iscoupled out of the respective optical cavities and into the opticalfiber 320.

FIG. 11 shows another embodiment of the dual swept optical source 100,using laser sources, according to the principles of the presentinvention.

This embodiment differs from some of the previous embodiments in that ithas no shared optical cavities between the two tunable laser sweptsources 100-1, 100-2.

Separate lenses 416-1, 416-2 couple light from the laser cavities intoseparate fiber pigtails 268-1, 268-2 to separate birefringencecompensators 565-1, 565-2 and separate with partial reflectors 270-1,270-2. The first tunable optical signal 128-1 and the second tunableoptical signal 128-2 are combined in a fiber coupler that functions asthe combiner 200. One of the output ports of this combiner 200 is thencoupled to the optical fiber 320 that transmits the combined opticalsignal to the OCT interferometer. Preferably, the other port of thecombiner 200 is coupled to the k-clock 250.

This embodiment has the advantage that the separate birefringencecompensators 565-1, 565-2 are individually adjustable in order tocompensate for birefringence for each of the tunable laser swept sources100-1, 100-2 separately. Further, the use of the separate opticalcavities and in particular the separate fiber pigtails 268-1, 268-2allows for the individual adjustment of the cavity lengths of thetunable laser swept sources 100-1, 100-2.

FIG. 12 shows another embodiment of the dual swept optical source 100,using laser sources, and including an integrated laser clock system 250that has been constructed according to the principles of the presentinvention.

Generally the integrated dual laser clock system 100 comprises a twotunable laser subsystems, which generates a wavelength or frequencytunable first and second optical signals, and a clock subsystem 250,which generates clock signals at equally spaced frequency increments asthe tunable signals or emissions of the laser 100 are spectrally tunedover a spectral scan band(s). The clock signals are used to triggersampling of the analog to digital converter subsystem 315.

The tunable dual laser subsystem 100, combiner 200, and clock subsystem250 of the integrated laser system 100 are integrated together on acommon optical bench B. This bench B is termed a micro-optical bench andis preferably less than 20 millimeters (mm) by 30-50 mm in size so thatit fits within a standard or near standard butterfly or DIP (dual inlinepin) hermetic package 500. In one implementation, the bench B isfabricated from aluminum nitride. A thermoelectric cooler is disposedbetween the bench B and the package 500 (attached/solder bonded both tothe backside of the bench B and inner bottom panel of the package 500)to control the temperature of the bench B.

In the current implementation, the tunable lasers each comprisesemiconductor gain chip 410-1, 410-2 that is paired with amicro-electro-mechanical (MEMS) angled reflective Fabry-Perot tunablefilters 412-1, 412-2 to create external cavity laser (ECL) with thetunable filters being an intracavity tuning element and forming one end,or back reflector, of laser cavities.

The semiconductor optical amplifier (SOA) chips 410-1, 410-2 are locatedwithin their respective laser cavities. In the current embodiment, bothfacets of the SOA chip 410-1, 410-2 are angled relative to a ridgewaveguide. The SOA chips 410 are mounted on submounts S that, in turn,are mounted on the top side of the optical bench B, typically by solderbonding.

To collect and collimate the light exiting from each end facet of theSOAs 410-1, 410-2, lens structures 414-1, 414-2 and 416-1, 416-2 areused. Each lens structure comprises a LIGA mounting structure M, whichis deformable to enable post installation alignment, and a transmissivesubstrate U on which the lens is formed. The transmissive substrate U istypically solder or thermocompression bonded to the mounting structureM, which in turn is solder bonded to the optical bench B.

The first lens components 414-1, 414-2 couple the light between theinput facet of the SOAs 410-1, 410-2 and the tunable filters 412-1,412-2. Light exiting out the output facets of the SOAs 410-1, 410-2 arecoupled by lens component 416-1, 416-2 to optical fiber 320 via itsfront facet. The optical fiber pigtail 268 that leads to thebirefringence compensator and external cavity reflector 270 is alsopreferably solder attached to the optical bench B via a mountingstructure 330.

In more detail, fold mirror 254 directs the second optical signal tocombiner 252, which is configured according to one of the previousdescribed options, through a halfwave plate 262, if required. Thecombined beam is then coupled into optical fiber 268 by lens 261 to theoptical fiber facet that is held by mounting structure 330.

The light transmitted by the tunable filters 412-1, 412-2 is coupled outof the laser cavities and into the clock subsystem 250 to be collimatedby a third lens component 280-1, 280-2 and fourth lens components 282-1,282-2 for each tunable laser, which are solder bonded to the opticalbench B.

Fold mirror 286 and beam combiner 288 combine the beams from eachtunable laser into a common beam. The light then passes through a beamsplitter 290, which is preferably a 50/50 splitter to a clock etalon292. Any light reflected by the splitter 290 is directed to a beam dumpcomponent 294 that absorbs the light and prevents parasitic reflectionsin the hermetic package 500 and into the laser cavities.

The clock etalon 292 functions as a spectral filter. Its spectralfeatures are periodic in frequency and spaced spectrally by a frequencyincrement related to the length and refractive index of the constituentmaterial of the clock etalon 292, which is fused silica in one example.The physical length of etalon 292 is L. The etalon can alternatively bemade of other high-index and transmissive materials such as silicon forcompactness, but the optical dispersion of the material may need to becompensated for with additional processing inside the DSP. Also, air-gapetalons, which are nearly dispersionless, are another alternative. Stilla further alternative is a free-space interferometer (e.g. a Michelson),which also is dispersionless and is adjusted by simply moving therelative positions of the mirrors.

The contrast of the spectral features of the etalon is determined by thereflectivity of its opposed endfaces. In one example, reflectivity atthe etalon endfaces is provided by the index of refraction discontinuitybetween the constituent material of the etalon and the surrounding gasor vacuum. In other examples, the opposed endfaces are coated with metalor preferably dielectric stack mirrors to provide higher reflectivityand thus contrast to the periodic spectral features.

In the illustrated example, the clock etalon 292 is operated inreflection. The light returning from the clock etalon 292 and reflectedby beamsplitter 290 is detected by detector 298. The light detected bydetector 298 is characterized by drops and rises in power as thefrequency of the tunable signal scans through the reflectivetroughs/reflective peaks provided by the clock etalon 298. Lighttransmitted by the etalon 292 is collected by beam dump 295.

FIG. 13 shows another implementation of the k-clock 250 that allows forthe simultaneous monitoring of both the first tunable optical signal andthe second tunable optical signal from the swept sources.

In more detail, input beam 260 from the beam combiner 200 is reflectedby a fold mirror 291. The light then passes through a beam splitter 290,which is preferably a 50/50 splitter to a clock etalon 292. Any lightreflected by the splitter 290 is directed to a beam dump component 294that absorbs the light and prevents parasitic reflections in thehermetic package 500 and into the laser cavities.

As described previously, the clock etalon 292 functions as a spectralfilter. Its spectral features are periodic in frequency and spacedspectrally by a frequency increment related to the length and refractiveindex of the constituent material of the clock etalon 292, which isfused silica in one example.

In the illustrated example, the clock etalon 292 is operated inreflection. The light returning from the clock etalon 292 and reflectedby beamsplitter 290 is received by a splitter element 299. In asituation where are the first tunable optical signal and the secondtunable optical signal operate in different spectral scan bands, thesplitter element is a WDM filter that transmits the light associatedwith the first tunable optical signal to a first detector 298-1 andreflects light associated with the second tunable optical signal to asecond detector 298-2. In this way, the detectors 298-1, 298-2 are usedto generate separate k-clock signals simultaneously for each of thefirst tunable optical signal and the second tunable optical signal.

FIG. 14A illustrates the scanning of a conventional swept source. Thesource sequentially a tunes through the wavelength scan band S. This isthe repeated in serial scans. The problem is that the scans cannot beperformed in close time spacing without going to an different systemconfiguration such as ring laser cavities with polygon filters. Insteadthere is a gap between successive scans S associated with the retraceperiod R. While it is possible to scan in both directions, this istypically not feasible since semiconductor gain elements preferably onlyscan in one direction due to the Bogatov effect. In addition, themechanical inertia of the MEMS filter prevents a quick resweep at hightuning speeds.

FIG. 14B illustrates the scanning of dual swept source according to oneexample under control of the tuning controller 125. Here, a first ofoptical signal is scanned as illustrated by scan S1. Then, a secondtunable optical signal is scanned as illustrated scan S2. The process isthen repeated for subsequent scans. In this embodiment, the scans foreach of the two of optical signals occur over the same wavelength scanbands. The duty cycle is in effect doubled. In this example, a combiner200 that implements a beam switch can be used due to the timemultiplexed nature of the tunable optical signal scans S1, S2.Alternatively, the first swept source and the second swept source arealternately energized such that only one is emitting at any given time.As such, a partially reflecting element or mirror is used as thecombining element 252, in the simplest implementation.

This scanning configuration also enables speckle averaging with twoconsecutive A-lines (polarization diversity or not). The speckle patternis different for different scan times of biological samples.

FIG. 14C illustrates the scanning of the dual swept source according tostill another example. Here each of the scans S1, S2 have differentpolarizations as indicated by the inset arrows. Thus while in thisexample a beam switch combiner 200 could be used, in the preferredembodiment, the combiner 200 utilizes a polarization beam combiner.

FIG. 14D illustrates still another example in which these successivescans S1, S2 associated with the first tunable optical signal and thesecond tunable optical signal occur over contiguous scan bands.Specifically scan S1 occurs at a shorter wavelengths whereas scan S2occurs over a range of longer wavelengths.

FIG. 14E illustrates still another example in which these successivescans S1, S2 associated with the first tunable optical signal and thesecond tunable optical signal occur over non-overlapping scan bands.Specifically scan S1 occurs at a shorter wavelengths whereas scan S2occurs over a range of longer wavelengths that are separated from thescan band of S1 by a guard band. G.

FIG. 14F illustrates still another example in which these successivescans S1, S2 associated with the first tunable optical signal and thesecond tunable optical signal occur over non-overlapping scan bands, andsimultaneously in time. Specifically scan S1 occurs at shorterwavelengths whereas scan S2 occurs over a range of longer wavelengthsthat are separated from the scan band of S1 by a guard band. G. Thescans occur simultaneously with each other using a combination of a WDMcombiner 200 and WDM separation at detection.

FIG. 15 shows an optical coherence analysis system 300 using theintegrated multiple swept source system 100, which has been constructedaccording to the principles of the present invention.

The previous examples focused on dual swept source systems. However, inalternative examples, more than two swept sources are included in theintegrated swept source system 100. This embodiment illustrates a sourcesystem 100 with four swept sources: a first swept source 100-1, a secondswept source 100-2, a third swept source 100-3, and a fourth sweptsource 100-4. Each of these individual swept sources generatesrespective tunable optical signals, a first optical signal, a secondoptical signal, a third optical signal, a fourth optical signal, whichare combined into the combined optical signal by a combiner 200 andcoupled onto the optical fiber 320.

Using four sources has the advantages of higher speed, higherresolution, and 100% duty cycle when using a two detectors system set upas shown in FIG. 2, one for each band. Further, higher speed andpolarization sensitivity with four sources using two sources with onepolarization and two with another is possibility.

FIG. 16A illustrates the scans S1, S2, S3, S4 associated with the firsttunable optical signal, the second tunable optical signal, the thirdtunable optical signal, the fourth tunable optical signals generated bya first swept source 100-1, a second swept source 100-2, a third sweptsource 100-3, and a fourth swept source 100-4, respectively. In thisexample, the scans S1, S2 take place over a first scan band with 100%,or near 100% duty cycle. Likewise scans S3, S4 take place over a secondscan band with 100%, or near 100% duty cycle.

FIG. 16B illustrates the successive scans S1, S2, S3, S4 associated withthe first tunable optical signal, the second tunable optical signal, thethird tunable optical signal, the fourth tunable optical signalsgenerated by a first swept source 100-1, a second swept source 100-2, athird swept source 100-3, and a fourth swept source 100-4, respectively.In this example, the scans S1, S2, S3, S4 occur over non-overlappingscan bands, and simultaneously in time.

FIG. 17 illustrates an alternative scanning regime. Here the S1 scansfrom the first optical signal is at a higher scan rate than the scans S2of the second tunable optical signal. In particular, the scans S1associated with the first tunable optical signal are relatively fast,twice as fast in the illustration, than the spectral scans of the secondtunable optical signal S2. This embodiment is useful when differentpathlengths are being investigated by the separate tunable opticalsignals in the sample. In other examples, the slower scan is associatedwith the spectral analysis of the sample where is the higher rate scanis associated with the OCT analysis of the sample. This dual modalityanalysis is used for example in the hybrid spectral/OCT analysis systemof FIG. 3.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims. For example, although the inventionhas been described in connection with an OCT or spectroscopic analysis,the invention could also be applied along with IVUS, FLIVUS, HIFU,pressure sensing wires and image guided therapeutic devices.

What is claimed is:
 1. An optical coherence analysis system comprising:a first swept source that generates a first optical signal that is tunedover a first spectral scan band, the first swept source including afirst semiconductor optical amplifier and a first tunable filter fortuning light from the first semiconductor amplifier over the firstspectral scan band; a second swept source that generates a secondoptical signal that is tuned over a second spectral scan band, thesecond swept source including a second semiconductor optical amplifierand a second tunable filter for tuning light from the secondsemiconductor amplifier over the second spectral scan band; a combinerfor combining the first optical signal and the second optical signal toform a combined optical signal; an interferometer for dividing thecombined optical signal between a reference arm leading to a referencereflector and a sample arm leading to a sample; a multi channel k-clocksystem for detecting the first optical signal and the second opticalsignal to generate clock signals, wherein the multi channel k-clocksystem receives the combined optical signal and separately detects thefirst optical signal with a first detector to generate a first clock andthe second optical signal with a second detector to generate a secondclock as the clock signals; a detector system for detecting aninterference signal generated from the combined optical signal from thereference arm and from the sample arm; and an analog to digitalconverter system that samples the interference signal and also receivesthe clock signals.
 2. A system as claimed in claim 1, wherein thecombined optical signal is coupled into a spectrally filtering element.3. A system as claimed in claim 2, wherein the spectrally filteringelement is an etalon.
 4. A system as claimed in claim 1, furthercomprising an optical bench, on which the first swept source, secondswept source and the multi channel k-clock system are implemented.
 5. Asystem as claimed in claim 1, wherein the multi channel k-clock systemcomprises a beam splitter for dividing the first optical signal from thesecond optical signal and providing the first optical signal to thefirst detector and the second optical signal to the second detector,respectively.
 6. A system as claimed in claim 5, wherein the beamsplitter is a WDM filter.
 7. An optical coherence analysis methodcomprising: generating a first optical signal that is tuned over a firstspectral scan band with a first swept source including a firstsemiconductor optical amplifier and a first tunable filter for tuninglight from the first semiconductor amplifier over the first spectralscan band, wherein components of the first swept source are attached toan optical bench; generating a second optical signal that is tuned overa second spectral scan band with a second swept source including asecond semiconductor optical amplifier and a second tunable filter fortuning light from the second semiconductor amplifier over the secondspectral scan band, wherein components of the second swept source areattached to the optical bench; combining the first optical signal andthe second optical signal to form a combined optical signal; dividingthe combined optical signal between a reference arm leading to areference reflector and a sample arm leading to a sample; detecting thefirst optical signal and the second optical signal to generate clocksignals by receiving the combined optical signal and then separatelydetecting the first optical signal to generate a first clock with afirst detector and the second optical signal to generate a second clockwith a second detector; and processing an interference signal generatedfrom the combined optical signal from the reference arm and from thesample arm in response to the clock signals.
 8. A method as claimed inclaim 7, wherein the combined optical signal is coupled into aspectrally filtering element.
 9. A method as claimed in claim 8, whereinthe spectrally filtering element is an etalon.
 10. A method as claimedin claim 7, further comprising generating the first optical signals, thesecond optical signals, and the clock signals on an optical bench.
 11. Amethod as claimed in claim 7, further comprising dividing the firstoptical signal from the second optical signal prior to detection by theseparate detectors.
 12. A method as claimed in claim 11, furthercomprising the first optical signal from the second optical signal witha WDM filter.
 13. A system as claimed in claim 1, wherein the multichannel k-clock is attached to the optical bench.
 14. A method asclaimed in claim 7, wherein the clock signals are generated by a k-clocksystem that is attached to the optical bench.